After the debacle at Cannae, Scipio Africanus has discovered a plot among some Roman nobles to desert Italy. He has hurried to them, and, waving his sword around, threatened them with death unless they promised to desist from their treachery.

"Promise," he said, "you lot, that neither will you desert the Roman republic, nor suffer any other Roman to desert; let him who will not swear know that he will feel this naked blade in himself." Just as terrified as though they were seeing the conquering Hannibal, they all swear and yield to Scipio those who should be locked up, themselves themselves.

I can't work out why there are two selves - semet and ipsos. Are they both necessary? It seems like duplication to me. Do they all refer to the same people? semet can't possibly be nominative, can it? They themselves (the plotters) yielded the actual people that should be locked up to Scipio. But I think I'm clutching at straws there. And I can't decide if the custodiendos refer to the actual plotters, or whether they are dobbing other people. And I'm not sure if omnes goes with iurant, or with custodiendos. Other than that, I've pretty much got this sentence nailed!

Or maybe something like "...they all [so] swear and surrender themselves to Scipio, in order to be held in custody."

Yes, it's a duplication, much similar in fact to the English word "themselves" ("them"+"selves" : "se(met)"+"ipsos"). The difference is of course that this duplication is essential in English to express the reflexivity, while "se" alone in Latin is the reflexive pronoun, and the duplication is intensifying.

In a translation of Lhomond's Viri Romae by Edward Roth (1898) that I came across some time ago, it says:

"Swear," he cried, "swear that you will neither desert Rome yourselves nor allow any Roman citizen to do so! Whoever does not take this oath must be informed that it is against himself that this sword is drawn." Just as much frightened as if they saw the victorious Hannibal himself, they all took the oath and surrendered themselves to Scipio to be kept in custody.

By the way, I notice that the conjoined form 'semetipsos' (they themselves) is quite common when googled. As in:

... jurant omnes, custodiendosque semetipsos Scipioni tradunt.

Or as in Tertullian's grisly thoughts concerning De Resurrectione Carnis: